The type of power transmission apparatus of the type to which the present invention pertains is found in U.S. Pat. No. 4,795,014 entitled "Power Transmission Apparatus" which is incorporated by reference as if fully reproduced herein. The aforementioned patent discloses a power transmission apparatus wherein power is transmitted between a camshaft and a rotating cylinder block through a pair of pistons reciprocally and slidably disposed within a pair of adjacent cylinder heads in the cylinder block. The interiors of the pair of adjacent cylinder heads communicate with one another through a pair of sliding plates disposed therebetween which serve as valves to regulate the flow of hydraulic material. Sliding of the plates provides orifices of variable size. By sliding the plates to reposition the valves, the size of the orifices defined by the plates is variable to change the power transmission apparatus between a first mode in which a direct connection is provided between the camshaft and the cylinder block whereby the pistons are prevented from moving relative to the cylinder block and accordingly rotate therewith, and a second mode in which the relative speeds of the camshaft and the cylinder block are varied with respect to one another.
More specifically, the valves used in regulating the flow of hydraulic material in the aforementioned patented device comprises a pair of plates which are slidably disposed within channels defined in associated cylinder heads. Each plate has an elongated central opening which is greater in length than the distance between the pair of cylinders. In an open position, the opening in the slidable plate conmnunicates with both cylinders to permit flow of hydraulic material between the two cylinders. In a closed position, the opening in the slidable plate communicates with only one of the cylinders, thus preventing flow of hydraulic material between the two cylinders.